User 1 can also only see media uploaded my himself in the image browser. Media uploaded by every other user is not shown. On admin/content/media/thumbnails every image is visible for everyone, but they cannot be (re)used in practice.

I've been checking lots of settings, but can't find any clues there...

I am having a similar issue. I am only logging in with user 1 but some of the images I have uploaded do not appear in the Library when I am using the media file selector. They are visible when I go to content -> media and they definitely exist on the system.

Is there any insight or progress in this matter?
This issue makes the module more or less useless. Users are now forced to re-upload images, since they are not appearing in their finder when editing or adding nodes.

I'm seeing the same problem of only the latest 15 uploaded images (3 rows of 5) appearing in the media library on a "node/add" page... even though I have far more than that uploaded and visible at /admin/content/media

What types of files or extensions is your *field* configured to display? Is it possible there is a limit to what can be added to the field, and the Media browser only shows those files rather than all of them?